It is obvious to many of us, whether we are only children or not, that siblings are often different from each other.
Judy Dunn, emeritus professor of developmental psychology at Kings College London, and Robert Plomin, professor of behavioral genetics at the same institution, were among the first researchers to start asking empirically why this happens. Drawing on the differences they’ve noticed in Dunn’s children, over the past 30 years, they tested sibling character and personality variations.
Finally, it seems that they – and other experts in the field – have come closer to answering the question of why even identical twins who live under the same roof can sometimes be completely different characters.
How genetically similar are siblings?
Genetics usually predicts how siblings will be different. “Because you and your brother are 50 percent genetically similar, that means you’re also 50 percent genetically different,” Plomin says.
If it were that simple, the siblings would be half different and half the same. But genetics is not so linear. All genes are not transmitted in the same way since they undergo certain rearrangement throughout the reproductive process.
Learn more: Identical twins: how genetically alike are they?
Why are siblings different?
And few traits are predicted by a single gene: Even a trait as simple as eye color is dictated by a combination of genes working together. While eye color is actually a result of about 16 different genes mixed together, personality traits are determined by an even greater number of meticulously linked genes in varying combinations and formulas. Change even one gene and the result will be completely different. Thus, personality is less heritable than other traits.
“The more a trait is hereditary, such as height, the smaller the differences between siblings are on average. Weight, again, is surprisingly hereditary,” says Plomin. “But siblings are a bit more different when it comes to cognitive abilities, which are less hereditary than weight and height, and even less similar when it comes to personality and psychopathology.”
Where does all the difference come from?
Genetics helps explain the similarities between siblings, but not so much the differences. Environmental factors may explain the rest. Children growing up with the same two parents, who attend the same schools, can potentially experience radically different environments, both subjectively and objectively.
“In families, the environment works differently than we thought it would because children in the same family are different from each other. It’s actually a non-shared environment,” says Plomin. Not to mention the differences that naturally arise from extra-familial relationships: a teacher or a lover can completely change someone’s life.
What about parental favoritism?
The way parents treat their children can also have an impact on their character. A team led by Susan Marie McHale, professor of human development and family studies at Penn State University, says that from the outset, two siblings cannot live their parents in the same phase of their livesbe it romantic, financial or personal.
As a first or second child, perspectives can easily change and research has shown that it is not systematic. Not all older siblings have certain traits – they just have different experiences because they’re older.
Character differences in children lead to character differences in their caregivers. “We find that when parents have children of mixed gender, such as a boy and a girl, they are more likely to believe that their children have more influence over how they are treated,” says McHale. The same thing happens for adolescent parents versus children, she says.
At the end of the day, parents also have prejudices, preconceived notions and beliefs about their children and treat them a little differently – even though not all parents say they do.
Learn more: The Secret Reason Parents Play Favorites
Self-fulfilling prophecy and notes
Research by McHale published in the Journal of Family Psychology, for example, suggests that when parents think one child is more academically gifted than the other, that child’s grades improved more than those of his siblings. It wasn’t the other way around though. Sometimes it almost becomes a self-fulfilling prophecy, where slight differences between siblings are exaggerated by their parents.
In some cases, it can help understand differences between siblings, Plomin says. But statistically, these differences do not seem to be a sufficient correlation between all parental behaviors and all the personalities of their children.
In his book, Blueprint: How DNA Makes Us Who We Are, Plomin always suggests that although parents matter a lot in the character of their children, due to genetics, children are always predisposed to have certain personalities and abilities regardless. A parent’s job is to encourage their children to become the best version of themselves, he says.
Do siblings influence each other?
There’s a lot to understand about how siblings influence each other. “In a sibling relationship, you have different roles and different experiences as the giver and receiver of interactions and observations,” McHale explains.
In some cases, siblings seem to be an important influence on how adolescents act in risky situations, for example, how much they drink and whether they use substances. In other cases, they bring out the opposites of each other. “Your brother may be a role model or may be a foil,” says McHale.
Competition – and even jealousy – between two siblings can bring out similar or different personalities. It is in line with Darwin divergence principle.
Everything can be taken into account
Since none of these theories seem to yield satisfactory results, after 30 years of trying to identify systematic sources of sibling differences, Plomin argues that the question needs to be completely reframed.
“Part of the problem is asking what is the cause of something? said Plomine. “The answer is that these are complex developmental phenomena. The answer is probably thousands of things and different things for different people.
And chance and random experiences play a much larger role in our lives than people tend to believe. “I think these kinds of idiosyncratic, stochastic, or random differences are a big reason siblings are so different,” Plomin says.
Learn more: Picky Eating: Can We Blame Genetics?